FIG. 1 illustrates a single longitudinal mode tunable laser resonator with a prism inserted. The conventional method forms a resonator by including a wholly reflecting mirror 1, a laser gain medium 2 and a grating 3, and by shifting the wavelength. Among the beams which are diffracted by the grating 3, the beams which are incident perpendicularly to a tuning mirror 5 to be reflected are made to return to be amplified, thereby realizing laser oscillations.
In this case, the diffraction angles of the grating 3 are variously different depending on the wavelength. Therefore, the tuning mirror 5 is rotated, so that the wavelength of the beams perpendicularly reflected from the tuning mirror 5 would become different, thereby shifting the wavelength.